Conventionally, a variety of image forming apparatuses such as a copying machine have been proposed. Such image forming apparatuses are roughly divided into two types by a difference in transfer system of a color toner image onto a recording medium. One is an image forming apparatus adopting a so-called direct transfer system in which a recording medium is held by being wrapped around a transfer medium, and a toner image on a photoreceptor is directly transferred onto the recording medium color by color. The other is an image forming apparatus in which a color toner image formed on a photoreceptor is transferred at once onto an intermediate transfer medium, and thereafter transferred onto a recording medium from the intermediate transfer medium.
However, in the former image forming apparatus, there is a case where a desired color image cannot be obtained depending on the type of a recording medium used. This is because of the fact that the transfer characteristics of the toner image are different for different colors depending on the type (especially thickness) of the recording medium used. As a result, in a color toner image composed of overlapping toner images of different colors, the coloring differs among recording media. Thus, in order to obtain a desired color image in this image forming apparatus, it is required to limit the range of thickness, etc., of the recording medium used.
However, in recent years, a demand for color copying for a recording medium of various thicknesses and various paper types has come to a level which cannot be met by the described image forming apparatus. Further, in the above image forming apparatus, because the toner image is transferred to the recording medium color by color, the overlapping accuracy of the toner images is always a problem. However, it has come to a level where the overlapping accuracy cannot be improved any further.
Therefore, it now has been a focus of ongoing research to develop an image forming apparatus provided with an intermediate transfer medium, which is capable of color copying regardless of the type of a recording medium used. As an example of such an image forming apparatus, the image forming apparatus as disclosed in Japanese Unexamined Patent Publication No. 251864/1991 (Tokukaihei 3-251864) is described below. In this image forming apparatus, a copying process is carried out for each of a high density portion and a low density portion of a single document image. As a result, a color toner image obtained by a single copying process is overlapped with another color toner image on the intermediate transfer medium, thus forming a single color toner image.
The above image forming apparatus as a copying machine as disclosed in the above publication is provided with, as shown in FIG. 27, a transparent document plate 101 on an upper surface. Below the document plate 101 is provided an exposure optical system 103 for exposing and scanning a document 102 and exposing a photoreceptor 104 (mentioned later).
The exposure optical system 103 is provided with a light source lamp 103a for projecting light onto the document 102 placed on the document plate 101, a plurality of reflecting mirrors 103b through 103f for guiding, as shown by the alternate short and long line in FIG. 27, the reflected light off the document 102 onto the photoreceptor 104, a focus lens 103g provided in the path of the reflected light, and a color separating filter composed of color filters of three primary colors of red, green, and blue.
Underneath the exposure optical system 103 is provided the photoreceptor 104 having a belt shape. The photoreceptor 104 is suspended between two rollers 105 and 106, which are placed with a certain gap therebetween, and the photoreceptor 104 is rotatably driven by a motor (not shown).
Around the photoreceptor 104 on the roller 106 side are provided, along with other members, a static charger 107 for charging the photoreceptor 104, a cleaning device 108 for removing toner remaining on the photoreceptor 104, and a screen filter 109 for splitting the reflected light off the document 102 into rays.
On the upper side of the photoreceptor 104 is provided a developing device 113 having three developer tanks 110 through 112 without contacting the photoreceptor 104. The developer tanks 110 through 112 store color developers of yellow, magenta, and cyan, respectively, which are complementary colors of the three primary colors of the color filters of the color separating filter 103h. The developer tanks 110 through 112 are provided with magnet rollers 110a through 112a, respectively, which give the respective color developers to the photoreceptor 104.
Below the photoreceptor 104 are provided sheet feeding cassettes 114 and 115 on top of the other having different sizes for feeding a recording sheet 130 as a recording medium. On the discharge sides of the sheet feeding cassettes 114 and 115, sheet feeding rollers 116 and 117 are provided, respectively. In front of the sheet feeding cassettes 114 and 115 are provided timing rollers 118 for temporarily stopping the recording sheet 130 so that the recording sheet 130 is supplied at a predetermined timing.
On the roller 105 side of the photoreceptor 104 is provided an intermediate transfer device 119. The intermediate transfer device 119 is composed of, along with other members, the intermediate transfer medium 120 having a belt shape, three rollers 121 through 123 for rotatably driving the intermediate transfer medium 120, a transfer charger 124 for transferring a toner image of each color component on the photoreceptor 104 onto the intermediate transfer medium 120, a transfer charger 125 for transferring a color toner image formed on the intermediate transfer medium 120 onto the recording sheet 130, a separating charger 126 for separating the recording sheet 130 from the intermediate transfer medium 120, and the cleaning device 127 for removing toner remaining on the intermediate transfer medium 120.
In the discharge direction of the intermediate transfer medium 120 are provided a transport belt 128 for transporting the recording sheet 130 and a fixing device 129 for fixing the color toner image onto the recording sheet 130.
When carrying out full-color copying in the described arrangement, first, the screen filter 109 is set aside from the exposure path and the exposure is started with respect to a high density portion.
Specifically, the light source lamp 103a projects light onto the document 102 placed on the document plate 101 so as to carry out optical-scan three times. The reflected light off the document 102 is incident on the color separating filter 103h via the reflecting mirrors 103b through 103d and the focus lens 103g, and is separated into color components by the color separating filter 103h. The reflected light separated into color components is then successively projected, via the reflecting mirrors 103e and 103f, onto the photoreceptor 104, which has been uniformly charged by the static charger 107, so as to expose the photoreceptor 104. As a result, an electrostatic latent image of each color component, corresponding to the document image is formed on the photoreceptor 104.
The electrostatic latent image of each color is made visible by being developed by the corresponding developers of yellow, magenta, and cyan of the developing device 113, which are complementary colors of the three primary colors of the color filters of the color separating filter 103h, and the electrostatic latent image becomes a toner image. Then, in the intermediate transfer device 119, the toner image of each color component is successively transferred onto the intermediate transfer medium 120 by the transfer charger 124 so as to be overlapped. This completes a single color toner image with respect to the high density portion, and a first copying process with respect to the high density portion is finished.
Then, for exposure of a low density portion, the screen filter 109 is introduced into the light path of the light from the exposure optical system 103, and the optical scan is carried out in the described manner. Namely, in the exposure with respect to the low density portion, the reflected light off the document 102 is projected onto the photoreceptor 104 after being split into rays by the screen filter 109, thus exposing the photoreceptor 104.
Then, the electrostatic latent image formed by exposure is developed into a toner image of each color component. The toner image formed in this manner is successively transferred onto the color toner image formed on the intermediate transfer medium 120 in the previous transfer process, thus forming another color toner image. In this manner, a complete color toner image is obtained from two color toner images, as obtained from the low density portion and the high density portion, overlapping with each another.
The color toner image formed on the intermediate transfer medium 120 is then transferred by the transfer charger 125 onto the recording sheet 130 which has been supplied from either one of the sheet feeding cassettes 114 and 115. The recording sheet 130 is then separated from the intermediate transfer medium 120 by the separating charger 126 and is guided to the fixing device 129 by the transport belt 128, and the color toner image is heat-fixed in the fixing section 129.
The fixing device 129 is usually provided with a heat roller for heat-fixing the toner image on the recording sheet 130. The surface temperature of the heat roller is controlled to be a set temperature by the ON/OFF operation of a heater lamp. FIG. 28 shows a normal fixing temperature curve when the heater lamp is turned on.
As shown in FIG. 28, when the heater lamp is turned on, the surface temperature of the heat roller gradually increases to the set temperature. When the set temperature is reached, the heater lamp is turned off, but the temperature continues to rise by the remaining heat. When the surface temperature drops below the set temperature, the heater lamp is turned on again. This process is repeated subsequently, and this results in overshoot in which the surface temperature fluctuates.
FIG. 29 shows a change in fixing ability with time. As shown in FIG. 29, the fixing ability is stable on the recording sheet 130 from the front end to the point in length corresponding to the periphery of the heat roller. However, the fixing ability abruptly decreases from the point past the periphery of the heat roller to the rear end of the recording sheet 130. The is because in one rotation of the heat roller, the heat of the heat roller is given off to the recording sheet 130 or the toner to be fixed.
In order to prevent this decrease in fixing ability, as shown in FIG. 30, it has been conventional practice to carry out a control so that the heater lamp is turned on just when the surface temperature of the heat roller starts to fall below the set temperature.
Incidentally, in the described copying machine, transfer of the toner image of each color component from the photoreceptor 104 to the intermediate transfer medium 120 is carried out by the corona discharge of the transfer charger 124. Likewise, transfer of color toner image from the intermediate transfer medium 120 to the recording sheet 130 is also carried out by the corona discharge of the transfer charger 125.
In this kind of corona discharge, the oxygen molecules in the atmosphere are ionized and ozone is generated. Generally, ozone is toxic, and in high concentration, damages the respiratory system, and even a trace amount, when inhaled for an extended period of time, is fatal. Thus, considering environmental friendliness, generation of ozone is not preferable.
From this point of view, the described copying machine, provided with two corona dischargers, which are a source of ozone, lacks consideration for environmental friendliness.
Also, in the described copying machine, the transfer voltage applied by the transfer charger 125 is constant regardless of the type of the recording sheet 130 used. Therefore, there is a case where desirable transfer is obtained in one recording sheet 130 while transfer failure results when another recording sheet 130 having a different thickness is used. Especially, when thin recording sheet 130 is used, there is a case where re-transfer (back-transfer) results, in which the color toner image transferred on the recording sheet 130 is transferred again onto the intermediate transfer medium 120 when removing the recording sheet 130 from the intermediate transfer medium 120. Thus, in the above copying machine, because the transfer voltage is constant, a desirable transfer characteristic in accordance with sheet type is not obtained, and as a result, the printing quality suffers.
Conventionally, a copying machine having a function of changing the transfer voltage in accordance with the sheet feeding cassette storing recording sheets has been available. However, even when the recording sheets stored in the same sheet feeding cassette have the same size, the basis weight (corresponding to thickness) may not be the same. Further, the recording sheets having the same size may be transparent or non-transparent. Thus, a transfer characteristic in accordance with sheet type is not realized even with this arrangement.
Also, in the conventional fixing mechanism, a control is carried out such that the heater lamp is turned on only when the toner image fixed on the recording sheet 130 exceeds the periphery of the heat roller and when the surface temperature of the heat roller starts to fall below the set temperature. In this case, the surface temperature of the heat roller does not reach the set temperature immediately, and therefore fixing from the point past the periphery of the heat roller to the rear end of the recording sheet 130 is carried out with the surface temperature of the heat roller below the set temperature. As a result, a toner image cannot be fixed uniformly on the recording sheet 130 from the front end to the rear end.